1.. Field of the Invention
The present invention relates generally to hybrid motor vehicles and, more particularly, to a battery control system for controlling the state of charge of a hybrid vehicle battery.
2.. Background of the Invention
Motor vehicle manufacturers are actively working to develop alternative powertrain systems in an effort to reduce the level of pollutants exhausted into the air by conventional powertrains equipped with internal combustion engines. Significant development efforts have been directed to electric and fuel-cell vehicles. Unfortunately, these alternative powertrain systems suffer from several disadvantages and, for all practical purposes, are still under development. However, “hybrid” vehicles, which are equipped with an internal combustion engine and an electric traction motor that can be operated independently or in combination with the internal combustion engine to provide motive power for the vehicle, offer a compromise between traditional internal combustion engine powered vehicles and full electric powered vehicles.
A hybrid vehicle is typically equipped with a relatively high voltage hybrid powertrain battery, for example a 340 V battery, which provides electrical power to the electric traction motor. Many hybrid vehicles are also equipped with a lower voltage battery, for example a 12 V battery, which provides power to various vehicle accessories, such as the vehicle radio, lights and other electrically operated equipment not powered by the hybrid powertrain battery. The charge level of the vehicle batteries is monitored according to an index known as the battery State of Charge, or SOC. The SOC is defined by a ratio of the amount of residual charge remaining in a battery relative to its full charge capacity. Presently, a battery's SOC may be measured using a combination of a measurement method utilizing a correlation between SOC and a battery's voltage-current characteristics at the time of charging (or discharging) and a measurement method utilizing an accumulation of charged and discharged amounts.
In many hybrid vehicles, a battery control system controls charging (and discharging) of a battery based on the SOC. A limitation of many hybrid battery control systems is that they fail to manage the lower voltage battery's charge level as it supplies power to the vehicle accessories, particularly when the internal combustion engine is turned off. Failure to manage the lower voltage battery may result in the battery becoming depleted. Accordingly, a need exists for an improved battery control system for a hybrid vehicle that monitors and controls a charged state of the lower voltage vehicle accessory battery to prevent the battery from becoming depleted.